The spring can get compressed so it doesn't have enough force to close hard enough to make a good seal. Take the thing apart and clean it, making sure that the sealing surfaces are clean and smooth. Stretch the spring apart to elongate it and put it all back together, making sure to push the end cap all the way onto the tube as far as you can get it. This can work for a few times, but eventually the spring gets weak and needs to be replaced.